President Francois Hollande said there was no doubt it had been a terrorist attack "of exceptional barbarity".

A major police operation is under way in the Paris area to catch the killers.

French President Francois Hollande: "This is an act of exceptional barbarism"

The satirical weekly has courted controversy in the past with its irreverent take on news and current affairs. It was fire-bombed in November 2011 a day after it carried a caricature of the Prophet Muhammad.

The latest tweet on Charlie Hebdo's account was a cartoon of the Islamic State militant group leader, Abu Bakr al-Baghdadi.

A witness filmed the scene as the sound of gunshots were heard

People had been "murdered in a cowardly manner", President Hollande told reporters at the scene. "We are threatened because we are a country of liberty," he added, appealing for national unity.

UK Prime Minister David Cameron said in a tweet: "The murders in Paris are sickening. We stand with the French people in the fight against terror and defending the freedom of the press."